Activated electrode for electric discharge lamp



March 24, 1959 P. DELRlEU 2,879,427

ACTIVATED ELECTRODE FOR ELECTRIC DISCHARGE LAMP Filed Jan. 16, 1956 0% R :l I I 22 lei/f 1 F793 ACTIVATED ELECTRODE FOR ELECTRIC DISCHARGE LAMP Pierre Delrieu, Paris, France, assignor to Societe Anonyme Pour les Applications de lElectricite et des Gaz Rares- Etablissements Claude-Paz 81 Silva, Paris, France Application January 16, 1956, Serial No. 559,424

Claims priority, application France December 22, 1955 9 Claims. (Cl. 313-185) impaired by the deposition, on the envelope thereof, of

particles sputtered off the electrodes and of material volatilized from the electrodes.

One object of the invention is to minimize these sputtering and volatilization by a novel constitution of the electrode, which constitution is easy to achieve.

Thisinvention is an improvement in the invention disclosed in my copending application Serial No. 535,331,

filed September 20, 1955, which application is assigned to the same assignee as the present invention. In said copending application I have disclosed a process for manufacturing a gaseous electric discharge lamp the discharge atmosphere of which has an absolute pressure higher than 0.5 kg. cm. during the operation thereof, the envelope of which is made of a vitreous material having a very high silica content, and at least one electrode of which .is electron-emissive and comprises a refractory metal member provided with electron-emissive material, said emissive material being obtained from an activating substance which has a melting point higher than 2500 C. and which produces, during the formation of said electrode, only products among which those which are not gaseous in the conditions of their production have melt ing points higher than 2500" C., these melting points being measured in the conditions, of pressure for example, to which these substance and products are subjected when the lamp is being manufactured.

United States Patent r In the above definition of the activating substance, the

the metal member in a condition of suspension in a liquid, and the degassing of the discharge lamp and the formation of the electron-emitting electrode is eiiected by heating the whole of the lamp to a temperature higher than 900 C., for'example by placing the lamp in an oven, while pumping it and/or flowing an inert gas through it, the electrode being at that moment located in the lamp.

The present invention relates to the case where the electrode manufactured by the above process includes a metal part devoid of electron-emissive material and, betweensaid part and the stem or envelope end supporting said electrode, a member of refractory metal offering capillary interstices adapted to contain emissive material and one end at leastof which opens directly in the dis- 2 refractory metal member is formed by the winding, in the form of a spiral, of one strip or of a plurality of strips, of one or more metals, electron-emissive material being disposed in interstices located between the turns of the winding and on portions of the external surface of the member.

Electrodes thus activated only over a part of their sur faces, have been proposed for high pressure mercury vapour discharge lamps. In such case, the portion not provided with a coating of electron-emissive material is closer to the other electrode of the lamp than the member carrying electron-emissive material. When such a lamp is started, the discharge starts between the members carrying emissive material, since, at that time, the decrease in voltage drop at the electrodes, due to the emissive material, largely compensates the increased distance between such members with respect to the portions not coated with emissive material. This discharge starts in the form of a glow discharge but is transformed almost immediately into an arc discharge. The pressure of the mercury vapour is still small when the are discharge begins.

When, later, the temperature of the lamp rises, the voltage drop in the positive column increases because ofthe increased pressure of the mercury vapour. Furthermore, the portions not coated with emissive material heat up by virtue of conduction of the heat evolved by the voltage drop at the electrodes on the members carrying electron-emissive material. ode spot settles, after some time, on the end of each one of the portions carrying no emissive material.

Owing to this transfer of the cathode spot, the electrodes vaporize very little. At the beginning of the running-up period, the discharge takes place on the members coated with emissive material, which, being activated, withstand well the ionic and electronic bombardments. During the normal operation of the lamp, the ends of the portions carrying no emissive material are raised to a sufficiently high temperature for them to emit electrons abundantly but not high enough for the tungsten of these portions actually to vaporize at the mercury vapour pressure then prevailing. As to the members provided with emissive material, they are not raised to such a temperature that they can be a source of vaporization. The cathode spot no longer occurs on them, and they are connected to the location of the cathode spot by the entire length of the portions carrying no electron-emissive material.

The construction according to the present invention, of the member provided with emissive material is such that said member will have a large heat capacity with a small bulk. Moreover this construction will be such that it offers the emissive material a large area for securing it, and that it makes it possible to locate the emissive material at the most favorable place for its preservation.

This construction allows the provision of thin electron emissive coatings, while securing a relatively important reserve of emissive material. These coatings being thin resist well to the forces due to the discharges.

In addition, this construction will be such that it will be possible to prevent the peripheral portion of the outer surface of said member from being coated with electronemissive material. Material if applied to such portion would gradually be dislodged by the discharge and would form stains on the Wall of the lamp.

This member is preferably threaded by a metal pin or wire, which pin projects beyond the member in the direccharge space. It is characterized by the fact that said tion of the incoming electric discharge. For example, the member is formed by one or more strips wound around the pin.

The invention will be better understood fromthe following description of preferred embodiments taken in Pa tented Mar. 24, 1959 For these two reasons, the cathconjunction with the accompanying drawings, in which:

'Figure's'l an'd2 show diagrammatically, in front view and side view respectively, an electrode.

Figure 3 represents amodifiedshape of the strip prior to Winding.

Figure 4 shows diagrammatically,.in side view, another electrode.

Figure shows diagrammatically a discharge lamp with two electrodes.

The electrode representedon Figures 1 and 2 comprises a strip 1 of refractory metal, .for example of tungsten, tantalum or molybdenum, Wound around arod 2 of a very refractory metal, such as tungsten. One end,

'3, of the strip 1 has been welded to the rod 2 .and the strip has been wound so as toconstitute the equivalent of a cylinder 10, the basesS and '9 .of which are formed by the edges of the strip. The windinglti offers a large heat capacity relatively to its outer area, which is advantageous since, in this manner, it does not heat up too much during the starting. In addition, said winding also offers a relativelylarge strip area, as compared with its volume, which makes it possible to arrange the reserve of emissive material in a thin and consequently adhering layer.

One end of rod 2, that is that shown on theleft in Figure 2, is adapted to constitute thesupportfor the electrode and it will be welded to a'thinmolybdenum strip which will be sealed vacuum-tightly in one stem or envelope end of the lamp. The other end 4 of rod 2 is rounded and it is on this end that-the cathode. spot will settle when the lamp operates under normal or substantially normal conditions.

In order to provide member with electron-emissive material, a suspension of activating substance is deposited on the ends 8 and/or '9 of the winding 10, which substance will produce the electron-emitting material. This suspension seeps into the interstices 5 of the winding whereafter it dries. The suspension is obtained, for instance, by crushing in a ball mill, for 60 hours, 100 grams of pure calcium carbonate, precipitated, with 100 grams of purified butyl acetate. After this crushing, about 250 grams of butyl acetate are added. With this dilution, the coatings obtained are sufiiciently adhesive although they contain no binder, and they contain enough calcium carbonate. Alternatively another suspension, containing a binder, may be employed, or a suspension incorporating calcium or thorium oxide instead of calcium carbonate. A small amount of the suspension remains on the ends 8 and 9 but the greater part seeps into the interstices 5 existing between the turns formed by the strip 1. There is thus obtained, after transformation of the activating substance into emissive material, a reserve of the latter, adhering and in a relatively large amount. It may be advantageous to place no suspension, either between the center turns close to the rod 2, or on the peripheral surface of the wound strip, that is, on the nearly cylindrical surface formed by the outer face of the last turn on the strip. The central zone is not provided with emissive material, since during normal operation this zone is the hottest in the winding 10, due to the proximity of the rod 2. The peripheral surface of member 10 is not provided with emissive material either, since said material would not be protected against bombardment at that place and would be rapidly pulverized.

Calcium carbonate and calcium oxide having high melting temperatures do not move when the electrode is being manufactured, nor later. They will not foul the surfaces which are not intended to be provided with electronemissive material.

'Experience hasshown that" the length ofthe. rod '2 at theright of the substantiallycylindrical member 10 con- "stituted by the wound strip should exceed a certainminimum for the cathode spot to pass 'from said member to the end 4 at a time when, on the one hand, .the member '10 has not reached-such a temperature that its emissive .material vaporizes substantially and when, on the other hand, the rod 2 is raised to a temperature sufiicient for it to emit enough electrons so that, taking into account the fact that the pressure of the mercury vapour is considerably higher, its surface will not be sputtered practically, when the discharge settles on it. If D be the distancefbetween theiend '4 of the rodj2 and the'face 9 provided with emissivematerial, a distance which is equal to the difference between the lengths of the shortest discharge pathsstar'ting from one point of the other electrodeand arriving, one at the portion 2 not provided with emissive material, and the other one at the member 10 provided with emissive material, then the distance D shouldbe such that, over a length of the positive column of the lamp equal to that distance, the voltage drop at normal operating'conditions.will be higher than 3.5 volts.

This distance between the end 4 and the member 10 offers the additional advantage thatthe latter is thus less heated .by the,positive column.

"On the other. hand,it.is necessary that the heat evolved at the end 4 by the cathode spot will, on the one .hand raisethis end to such attemperature that it emits electrons abundantly and on the other hand, will not heat .the

member 10 toomuch by conduction, which would cause a vaporization of the emissive material carried thereby, .and would cause a risk-of the cathode spot settling on such member. :It-has been found that, to obtain the desired effect, the current density iinthe rod 2 should exceed 6 .amp. per mm), at least at the beginning of the running-up,

the above condition regarding the difference in lengths D being assumed to be Ifulfilled.

If it is desired to :increase the amount of emissive material placed "as 'azreserve in the interstices 5 of the winding, protrusions may be produced on the strip 1. This increases the free space present between adjacent turns, while keeping the Winding of the strip pressed. It is well, however that the strip should ofier no protrusion on that portion of its length forming at least the first turn .from the-rod 2,.preferably about the two first turns, so as to facilitate the winding of the strip on the rod. Protrusions in that zone would reduce the number of turns comprised by the electrode,since the diameter of the latter must be smaller than the diameter of the quartz tube through which .it must pass-and which will servefor forming one end .or stem of the lamp. In Figure 1 the protrusions arerepresented by enlarged thickness of the spiral line representing, seen from one end, the surface separating the turns of the wound strip.

Figure 3 shows a modification of the shape of the strip 1. In thismodified embodiment, the strip includes accu- .tral portion of larger width between two points 11, 12 than the ends 3 and 6. When such a striphas been wound, .the central portion 11, 12 will form a projection onthe corresponding .face, 8 or 9 of the member 10. When .a drop .of suspension of activating material is deposited. on thernernber .1'0, itwill be easy to .apply it only to the projecting portion'and to avoid application to those end portions of the strip which form the central zone andthe periphery of the winding.

Figure 4shows diagrammatically, another electrode according tothe invention. This electrode comprises, in addition to the elements of the above electrode, a tungsten wire 7, tightly .wound. and with turns touching, on the rod.;2. Thelwinding 7, at itsright .hand end, is welded tothe'rod 2 sothat the end 4, on which the cathode spot forms. in-normal operatiomwill be common to the rod 2 and to the winding 7. The left hand end'of the winding 7 stops .at a distance from the face 9 of the member 10.

LThisconstructionrnrakes it possible togiVeLthe rod'i2 '-a.relatively-:small .diameter, thereby reducing the heating improves the cooling on the end4.

. ,For instance, in the case of a lamp with an inner diameter of 18 mm., operating normally with a current of 5 amperes and such a mercury vapour pressure that the potential gradient in the positive column is 9 volts per centimeter, an electrode may be used, of the type shown in Figures 1 and 2, having the following dimensions: the member comprises a winding of 6 to 7 turns of a strip of molybdenum 50 mm. long, 5 mm. wide and one tenth of a millimeter thick. This strip is wound around a tungsten rod 2, one millimeter in diameter, and forms approximately a cylinder 3.8 mm. in diameter; the length D is 7 millimeters, which is equal to the length of the positive column along which the voltage drop in normal operation is 6.3 volts. During passage to the steady operating state of the lamp, the cathode spot passes from the member 10 to the end 4 approximately when the current is 6 amperes and the potential gradient 5 volts per centimeter.

With another lamp, having an inner diameter 25 mm., operating normally with a current of 6.7 amperes and a mercury vapour pressure such that the potential gradient in the positive column is 10 volts per centimeter, an electrode may be used, of the type shown in Figure 4 and having the following dimensions:

The member 10 is constituted by a winding of about 7 turns of a molybdenum strip 50 mm. long, 5 mm. wide and one tenth of one millimeter thick. This strip is wound around a tungsten rod 2, 0.7 mm. in diameter. The length D is 7 mm., which is equal to the length of positive column along which the voltage drop, in normal operating conditions, is 7 volts. At the end of the rod 2, about ten turns of tungsten wire 7 are wound, such wire being 5 tenths of one millimeter in diameter. During the passage of the lamp to steady state operating conditions, the cathode spot passes from the member 10 to the end 4, approximately when the current is 7.5 amperes and the potential gradient 5 volts per centimeter. If the energizing circuit for the lamp had other characteristics, this transfer of the cathode spot could take place in other conditions, for instance with a current of 6.7 amperes and a potential gradient of 6.5 volts per centimeter.

The members 10 of the two electrodes may be provided with activating material by depositing, by means of a brush, a drop of a suspension of calcium carbonate, for example, at one point on the face 9, or better on the face 8 so as not to foul the rod 2 between its end 4 and face 9.

The suspension of calcium carbonate used may be that for which the composition has been indicated above, and the treatment to which these electrodes are subjected before the lamp is put in service may be the one already described, that is heating lamps containing these electrodes to a temperature higher than 900 C. (1100 C. when calcium carbonate is used), while pumping the gases out of the lamp.

If desired, it is also possible to use a suspension of calcium oxide, obtained, for example, by milling 100 grams of lime in 100 grams of butyl acetate for 6 to 50 hours, then diluting the mixture obtained in 500 grams of butyl acetate. Thorium oxide may also be used. The heating by the oven, through the envelope, may then, in some cases, not be quite so strong as when calcium carbonate is used.

The suspension of activating material may also contain a refractory metal, tungsten for instance, in very finely powdered form, or silica, which may increase the adherence of the emissive material. A binder may also be added, for instance 1% in weight of nitrocellulose with respect to the activating substance.

The member 10 may have other shape than those described above; it may consist, for instance, of a strip cut in a different manner, or of a plurality of strips, of the same metal or of different metals. Other shapes, again, may be adopted for the electrodes, within the scope of the present invention.

The coating with thesubstance whichwill produce the emissive material may be effected before welding the rod 2 to the thin molybdenum strip which will be sealed in an envelope end.

The discharge atmosphere may be krypton or xenon, with a pressure higher than 0.5 kg. :'cm. during operating.

Figure 5 represents diagrammatically a high pressure mercury vapor discharge lamp 23 comprising two electrodes 20, 22 and 30, 32 of the type represented on Figure 2. These portions of the outer surface of the electrodes which are coated with electron-emissive materials are indicated by dashes 21 and 31.

With the exception of the electrodes, this lamp is of a conventional model. Reference number 33 indicates a mercury drop the vapor of which will constitute the main part of the discharge atmosphere during the normal operation of the lamp.

What I claim is:

1. An electron-emissive electrode for an electric discharge lamp containing an ionizable atmosphere having an absolute pressure higher than 0.5 kg.:cm. during the oper ation of the lamp, the envelope of the lamp being made of a vitreous material having a very high silica content, said electrode including a metal part devoid of electronemissive material and, between said part and the envelope end supporting said electrode, a member of refractory metal offering capillary interstices containing electronemissive material and one end at least of which opens directly in the discharge space, said electron-emissive material being obtained from an activating substance which has a melting point higher than 2500 C. and which produces, during the formation of the electrode, only products among which those which are not gaseous in the conditions of their production have melting points higher than 2500 (3., said activating substance being deposited on said member in a condition of suspension in a liquid, and said member being formed by the winding, in the form of a spiral, of at least one strip made of at least one metal, the emissive material being disposed in interstices located between the turns of the winding and on portions of the external surface of the member.

2. An electrode as set forth in claim 1, wherein the spiral winding is provided with electron-emissive material obtained from an activating substance which is deposited as a suspension in a liquid on interstices between the successive turns of the winding.

3. An electrode as set forth in claim 1, wherein at least one strip constituting the member provided with emissive material offers protrusions, the portion of winding nearest the winding axis being devoid of protrusions at least over a length of strip corresponding to one turn of winding.

4. An electrode as set forth in claim 1, wherein the winding is threaded by a rod which serves to support at least a portion of the part devoid of emissive material, and the portion of the rod farthest from the winding is surrounded by a helix of refractory metal, said helix being devoid of electron-emitting material.

5. An electrode as set forth in claim 2, wherein the activating substance is deposited on portions of the winding other than the peripheral portion of the outer surface of the member and than a portion closest to the axis of the winding.

6. An electrode as set forth in claim 5, wherein no activating substance is deposited on at least the two turns of the winding closest to the axis thereof.

7. An elecrode as set forth in claim 2, wherein the strip width is greater in that portion of the winding on which the suspension of activating material is deposited than at the periphery of the winding and at the portion of the winding located in the zone closest to the Winding axis.

8. An electrode as set forth in claim 4, wherein said helix is wound tightly on the rod and with turns touching.

9. An electrode as set forth in claim 4, wherein said helix does not extend to the spiral winding.

(References on following page) References Cited in the file ofthis patent UNIT ED, STATES PATENTS Anderson et a1. June 25, 1935 Nauth Oct. 20, 1936 5 Francis Jan. 23, 1940 

